This study will improve our understanding of the normal physiology of leptin's secretion and its role in the neuroendocrine response to starvation in women. It has significant clinical implications for two states in particular: 1) low leptin states, such as anorexia or exercise induced amenorrhea, in which leptin administration may correct or decrease the hormonal and metabolic derangements and 2) high leptin states, such as obesity, which are treated with hypocaloric diets and may also benefit from leptin administration by permitting maintenance of weight loss and blocking intrinsic metabolic efforts to maintain a positive energy homeostasis. Hence, this study will elucidate the pathophysiology and may provide the basis for successful therapeutic options for these states. This study will have a placebo-controlled design. The subjects will be evaluated three times: once in the fed state, once in the fasted with leptin and once in the fasted with placebo. It is impossible to blind subjects and investigators regarding the studies in the fed state, but the two studies in the fasted state will be performed in a double-blinded randomized fashion. Each GCRC admission will be 4 days and will require blood and urine collection. Please see the research-training proposal in section FF for further details. [unreadable] [unreadable]